Particular strain gage orientation for a six component load measurement device

ABSTRACT

Disclosed is a particular method of strain gage placement on a cylindrical or tubular device for measuring the three force and three moment components of a load. The prescribed parameters of this method optimize measurement sensitivity of the gages and simplify production. On any calibrated cylindrical or tubular device (e.g., a prosthetic or robotic arm), at least six strain gages shall be attached and wired independently or in an independent bridge configuration. A first set of at least three gages shall be oriented approximately sixty degrees or less from either side of the long axis of the device. Then, a second set of at least three gages shall be oriented between approximately forty-five and one hundred-twenty degrees to the first set of gages, and approximately no greater than sixty degrees from either side of the long axis of the device. Each respective gage shall be equally spaced from each other around the periphery of the device. A computer or other such calculation means shall be employed to determine the components and compensate for drift and noise.

BACKGROUND OF THE INVENTION

A. Field of Invention

This invention relates to devices for measuring the three force andthree moment components produced by applying a load to such a device;specifically, measurement devices utilizing strain gages and theplacement and orientation of the strain gages on the measuring device.

B. Description of the Prior Art

Multi-component load transducers are typically designed by combiningvarious flexure elements into one body, where each flexure elementincorporates strain gages to measure a component of force or moment.

Alternatively, a cylindrical tubular design can be used where forces andmoments are measured by gages specially positioned and oriented toisolate the individual load components to be measured. (Cunningham andBrown, 1952) (Smith, 1970). The traditional method of isolating theindividual load components to be measured had the advantage where eachoutput of the transducer corresponded to one of the force or momentcomponents to be determined. Therefore, the conversion from measuredelectrical output signals to loads was a simple procedure.

Another approach measures the three force components transmitted throughthe neck of a hip prosthesis using three strain gages attached to theprosthesis with no specific orientation. (Bergmann, Journal ofBiomechanics, 1988). It would appear from the literature that the straingages are oriented and placed randomly. This results in a morecomplicated measurement conversion equation. Random orientation andplacement has been used in multiple devices. The theory is that, as longas each gage measures a quantity independent from the others, randomorientation and placement is possible. Unfortunately, random orientationand placement has the disadvantage that some channels may not have theiroptimum sensitivities.

After the advent of computers, difficult conversions as above arenormally done by software. Thus, complicated conversion equations nolonger pose a problem, yet the sensitivity problems remain.

The present invention eliminates the sensitivity disadvantages bypositioning one set of strain gages in approximately a less than sixtydegree orientation to the long axis of the cylindrical or tubular loadcell, and another set of gages approximately between forty-five and onehundred-twenty degrees from the first set and approximately less thansixty degrees from the long axis of the cell. This configurationoptimizes output sensitivities, and eliminates the need for precisionplacement of the gages. A computer or other calculation means shall beused to determine the components and compensate for drift and noise.

BRIEF SUMMARY OF THE INVENTION

The method of strain gage orientation and placement presently disclosedrelates to the measurement of the three force and three momentcomponents produced by applying a load to a cylindrical or tubular loadcell. This method offers significant improvements over the prior art byprescribing the orientation and placement of strain gages on themeasuring load cell. Such positioning increases and optimizes thesensitivity of the gages to the desired measurements. This configurationalso eliminates the need for precise placement of the gages on the cell,thus adding to the device's usefulness by reducing the costs associatedwith manufacturing.

The measuring device is comprised of a cylindrical or tubular load cellwith at least six strain gages attached to the cell. Alternatively, atleast six strain gages can be placed in bridge configurations on theload cell. The strain gages are positioned approximately equally spacedfrom each other around the periphery of the load cell. Further, theindividual gages are wired so that each gage is capable of independentmeasurement. Then, calculation means, such as a computer, are employedto determine the components of the load and compensate for drift andnoise.

The preferred embodiment is comprised of a first set of at least threegages, where the gages are oriented so that they are approximately lessthan sixty degrees from either side of the long axis of the cylinder.Then, another set of at least three strain gages is oriented so thatthey are between approximately forty-five and one hundred-twenty degreesto the first set of gages, and approximately less than sixty degreesfrom either side of the long axis of the cylinder. Therefore, withoutprecision positioning of the gages, and while overcoming the sensitivityproblems of the prior art, all six force and moment components can beaccurately measured.

Thus, the presently disclosed strain gage positioning method providesadvantages, improvements, and usefulness not contemplated by the priorart.

BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWINGS

FIG. 1 shows an exploded view of the cylindrical or tubular load cellwith strain gages oriented and placed within the parameters of thespecification.

FIG. 1 is for the Official Gazette.

DETAILED DESCRIPTION OF THE INVENTION

Presently disclosed is a method for measuring the three force and threemoment components of a load. It is comprised of a cylindrical or tubularload cell (e.g. a prosthetic or robotic arm), at least six strain gages,wiring, and accompanying calculation means. The purpose of thisdisclosure is to show a strain gage configuration upon the load cellthat overcomes the positioning and sensitivity needs of the prior artshown in the BACKGROUND section above.

In the preferred embodiment of the method FIG. 1, at least six straingages are mounted on a cylindrical load cell in sets of at least threegages per set. The strain gages are equally spaced from each other andare positioned around the periphery of the cell. Then, the gages areoriented to the long axis of the cylinder and oriented to each otherrespectively.

The first set of at least three strain gages FIG. 1A, which arepositioned approximately equally spaced from each other around theperiphery of the load cell, is oriented approximately less than sixtydegrees from either side of the long axis of the cylinder FIG. 1C. Forexample, FIG. 1A shows a strain gage positioned approximately parallelto the long axis. Such positioning, along the long axis of the cell FIG.1C, is the preferred positioning to produce optimum sensitivity.However, positioning outside the less than sixty degree parameters ofthis disclosure will significantly degrade sensitivity. Another set ofat least three strain gages FIG. 1B, which are positioned approximatelyequally spaced from each other around the periphery of the load cell, isoriented between approximately forty-five and one hundred-twenty degreesto the first set of gages FIG. 1A, and approximately less sixty degreesfrom either side of the long axis of the cylinder FIG. 1C. FIG. 1B showsthe second set of gages optimally positioned at approximately forty-fivedegrees from the first set of gages FIG. 1A. Such forty-five degreepositioning FIG. 1B is preferred to produce optimum sensitivity.However, positioning outside the one hundred-twenty degrees prescribedby this disclosure will significantly degrade sensitivity.

The positioning parameters disclosed simplify the production processbecause only approximate positioning is required; according to easilydetermined angles. Likewise, the sensitivity of the gages is maximizedwhen they are placed within the disclosed parameters, thereby producingimproved measurements. Therefore, without precision positioning of thegages, and while overcoming the positioning and sensitivity problems ofthe prior art, all three force and three moment components can beaccurately measured.

The strain gages of the preferred embodiment shall be wired so that eachgage is capable of independent measurement. Alternatively, otherembodiments and versions may be wired in six independent bridgeconfigurations.

In all embodiments and versions, calculation means, such as computers,shall be used to determine the components and compensate for drift andnoise.

The presently disclosed method does not require strain gage placement ona specific kind of cylindrical or tubular load cell. However, otherembodiments and versions shall continue to adhere to the abovepositioning parameters. The following is a nonexclusive listing ofapplications, embodiments, and versions.

Applied medically, these versions would position the gages as prescribedon cylindrical shaped prosthetics to measure the efficacy and loading onthe prosthetic device. Alternatively, when used commercially orindustrially, the disclosed gage positioning parameters allow for forceand moment measurements to be taken accurately; either in the field oron-site. For example, a construction or production crew can attach therequired number of gages to a machine, table, or any calibratedcylindrical object to determine the forces and moments the activity orload is exerting on that object.

Thus, the presently disclosed strain gage positioning method providesadvantages, improvements, and usefulness not contemplated by the priorart.

I claim:
 1. A cylindrical load cell having a longitudinal axis andcapable of measuring six load components, comprising a) at least twosets of strain gages, each set having at least three strain gagesattached to said load cell; b) said at least three strain gages in oneset of strain gages being approximately equally spaced from one anotherabout the periphery of said load cell and oriented between approximatelyzero to sixty degrees relative to either side of the longitudinal axisof said cylindrical load cell; c) said at least three strain gages of adifferent set of strain gages being approximately equally spaced fromone another about the periphery of said load cell and oriented betweenapproximately forty-five to one hundred and twenty degrees from the atleast three strain gages of the set of strain gages defined in paragraph(b) and between zero to sixty degrees relative to either side of thelongitudinal axis of said cylindrical load cell; d) each of said straingages in said at least two sets being wired to make an independentmeasurement operatively communicated to a computer processor tocalculate the load components and compensate for drift and noise.
 2. Acylindrical load cell having a longitudinal axis and capable ofmeasuring six load components, comprising a) at least two sets of straingages, each set having at least three strain gages attached to said loadcell; b) said at least three strain gages in one set of strain gagesbeing approximately equally spaced from one another about the peripheryof said load cell and oriented between approximately zero to sixtydegrees relative to either side of the longitudinal axis of saidcylindrical load cell; c) said at least three strain gages of adifferent set of strain gages being approximately equally spaced fromone another about the periphery of said load cell and oriented betweenapproximately forty-five to one hundred and twenty degrees from the atleast three strain gages of the set of strain gages defined in paragraph(b) and between zero to sixty degrees relative to either side of thelongitudinal axis of said cylindrical load cell; d) each one of saidstrain gages in said at least two sets being operatively connected to adifferent Wheatstone bridge configuration independently from one anotherto make an independent measurement operatively communicated to acomputer processor to calculate the load components and compensate fordrift and noise.